(1) Field of the Invention
The present invention relates to a process for making a preliminarily printed formed body. More particularly, the present invention relates to a process for making a draw-formed printed body by draw-forming a preliminarily printed metal blank, in which a printed image manifested on the side wall portion of a can is formed as a halftone printed image which is excellent in the dimension, size, distribution and arrangement of halftone dots and the reproducibility of this halftone printed image is improved.
(2) Description of the Related Art
Draw-forming of a surface-treated steel sheet such as a tinplate or tin-free steel sheet or a metal sheet such as an aluminum sheet is widely adopted for the manufacture of a so-called seamless can (two-piece can) and a metal cap. Coating or printing of individual formed bodies after the forming operation requires a complicated operation, and therefore, coating or printing of a flat blank before the draw-forming is desirable.
In the draw-forming, a flat metal blank undergoes the plastic flow and is formed into a cup-shaped body. When the plastic flow of the portion of the flat blank, that will constitute the circumferential side wall portion of the formed body, is taken into consideration, it is confirmed that in this portion, elongation is caused in the direction corresponding to the height direction of the formed body, while contraction is caused in the circumferential direction.
In the preliminary printing of a flat blank for draw-forming, in view of the above-mentioned plastic flow of the blank, it is necessary that an original which is rectangular in the developed state should be converted to an annular printing plate. As means for effecting this conversion, there are known a handwriting method, an optical conversion method and a digital conversion method proposed by us (Japanese Unexamined Patent Publication No. 61-267763 and Japanese Unexamined Patent Publication No. 61-267053).
According to the conventional halftone printing method for effecting this preliminary printing, an original image is inputted as a picture element signal into a plate-making computer, the inputted image is annularly developed by a deforming computer to form an annular image, the annular image is halftone-separated by an editing means to make a printing halftone plate, and a blank is printed by using this printing halftone plate.
However, in a formed body obtained by draw-forming a preliminarily printed metal blank, a striped pattern not found in the original print image is often formed at the upper part of the side wall portion, and even in the case where a simple original image is printed on the side wall portion of a can, the inherent impression of the halftone image is not found but a deformed image is observed, and a halftone printed picture having a good reproducibility cannot be obtained in the side wall portion of the can.
As the result of investigations made by us, it was found that in the draw forming of a preliminarily printed metal blank, at the upper part of the side wall portion, dots become continuous in the circumferential direction owing to the compression flow of the blank in the circumferential direction and spaces between dots in the height direction increase owing to the elongation flow of the blank in the height direction, and this deformation of the dots tends to result in formation of a striped pattern.